The present invention is related to improved solid electrolytic capacitors and more particularly to solid electrolytic capacitors comprising braided anode lead wires.
The electronics industry is constantly challenged to provide increased functionality in smaller packages. This trend, commonly referred to as miniaturization, has pushed component manufacturers to improve the volumetric efficiency of the products they produce. Though a general phenomenon, the instant application is primarily focused on solid electrolytic capacitors and improvements therein for increased volumetric efficiency.
Volumetric efficiency of a solid electrolytic capacitor is generally expressed as:Volumetric efficiency=Capacitance×Voltage/Volume.
Since capacitor manufacturers recommend different voltage derating guidelines, and these guidelines vary with temperature and other factors, the voltage is generally taken to be the manufacturer recommended maximum voltage for a particular operating condition. For capacitors, which are essentially a rectangular prism in shape, the volume is generally calculated from the maximum dimensions of the component along the x, y and z axes and for other shapes the geometric volume of the case is used as a reasonable approximation of the volume. There are a number of methods used to improve the volumetric efficiency of solid electrolytic capacitors. An important method for the discussion herein is the utilization of anodes formed as a monolith by pressing valve metal powders thereby providing for an increased surface area per unit volume.
The surface area of valve metal powders is often expressed as the specific charge of the powder. The relationship between specific charge and surface area of a valve metal powder is derived from the general equation (1) for capacitance:C=EkA/t  (1)wherein:    C=capacitance (farads);    E=permittivity constant (8.85×10−12 farads/meter);    k=dielectric constant;    A=anode/cathode surface area overlap (meter2); and    t=dielectric thickness (meter).
Given that t, the dielectric thickness, is proportional to the formation voltage employed to form the dielectric the expression for wet capacitance can be expressed as:C=EkA/Vf  (2)wherein:    Vf=formation voltage
Rearranging equation (2) yields:CVf=EkA  (3).
Since E and k are constants one can see that the product CVf is proportional to the surface area of the powder. The product CVf, divided by the weight of the powder, is referred to in the art as the specific charge, or the charge of the powder. Tremendous gains in the specific charge of tantalum powders has been achieved since the initial development of solid electrolytic capacitors in the 1950's. The graph in FIG. 6 illustrates the changes over a few decades. As the specific charge of a powder increases the sinter temperature decreases as also illustrated in FIG. 6.
Anodes for solid electrolytic capacitors are generally formed by etching a valve metal foil or pressing a powder of a valve metal to form a porous compact or monolith. For anodes formed from pressed powders, one or more lead wires are generally either embedded in the pressed compact or welded to an external surface of the compact. The lead wires are generally cylindrical in shape, although flat wires and ribbons have been proposed. The lead wires generally have the same chemical composition as the powder used to press the compact. For example, a tantalum lead wire is used with a pressed tantalum anode. After the anode is pressed it is sintered to form metallurgical bonds between the powder particles in the pressed compact. For lead wires which are embedded in the anode the powder also sinters to the lead wire.
As the sinter temperature increases the thickness of the necks between the powder particles, and between the powder particles and the embedded wire, increases and the charge of the powder decreases. Thus, lower sintering temperatures provide higher charge and higher volumetric efficiency of the finished device. This must be balanced by taking the robustness of the anode into consideration. If the sinter temperature is too low, the necks formed between powder particles and the embedded wire are easily broken during subsequent thermal excursions such as when the device is mounted to a circuit board. These broken necks result in short circuits during the board mount process. Capacitor manufacturers commonly use lead wire pull strength, the force required to pull the embedded wire out of a sintered anode, as a measure of the robustness of the anode. Low pull strength correlates with higher incidence of shorts at board mount. For high charge powders, where the sinter temperatures are inherently low, poor lead wire pull strength can be a major issue contributing to poor quality of the finished device.
As the volumetric efficiency of solid electrolytic capacitors increases the size of the anode decreases. Smaller anodes require smaller lead wires and shallower insertion depth. Both of these factors decrease the contact area between the anode and the embedded lead further reducing the lead wire pull strength and compromising product reliability.
Another factor driving the increased functionality of electronic circuits is increased frequency at which the circuit operates. High frequency circuits require solid electrolytic capacitors with low equivalent series resistance (ESR). The resistance of the lead wire and the connection between the lead wire and the anode contributes to the ESR of the device. Solid electrolytic capacitors employing multiple lead wires, flat wires and the like have been proposed in order to decrease these resistances.
Therefore, there is an ongoing need for improvements in the mechanical and electrical connection between the embedded anode lead wire and the anode. There also exists an ongoing need to reduce the resistance in the lead wire itself. These needs have not previously been met. Provided herein is an improved capacitor with increased mechanical and electrical connection between the anode and anode lead and a reduced resistance in the anode wire.